The present invention relates to a constant current circuit which generates a current having a predetermined value without temperature dependence or with predetermined temperature dependence, and which is suitable to be incorporated into an integrated circuit.
As is well known, a reference voltage is frequently used for precisely operating various electronic circuits, but it is also necessary in many cases to use a reference current for the same purpose as in the reference voltage. Of course, it is desired that this reference current should not be affected by variation of a power supply voltage, and also by a change of the temperature, as well.
At first, some conventional reference current sources suitable for generating reference currents having substantially no temperature dependence, will be briefly described below with reference to FIGS. 4(a) through 4(d), which show circuit configurations of the conventional reference current sources incorporated into a MOS integrated circuit.
FIG. 4(a) shows a current source circuit for a reference current without temperature dependence, which circuit utilizes an operational threshold value of a MOS gate (in detail, cf. P. E. Allen & D. R. Douglas: "CMOS Analog Circuit Design", published from Holt, Rinhart & Holberg Inc. in 1987, pp. 246-249). This circuit is composed in combination with a current mirror circuit including three p-channel transistors 61a to 61c and a reference circuit including two n-channel transistors 62a and 62b. While the mirror circuit on the power supply side is supplying currents to a resistor r and both transistors 62a and 62b, gate and source of which are connected with one another, an output current Io is taken out from the transistor 61c on the driven side.
FIG. 4(b) shows a self-bias type reference current source using a voltage between a base and an emitter of a parasitic transistor for a reference (Cf. P. R. Grey & R. G. Mayer, "Analysis and Design of Analog Integrated Circuit", the Japanese translation published from Baifukan Publishing Co., in 1990, pp. 307). This circuit is composed of the above mentioned mirror circuit including the transistors 61a to 61c, another mirror circuit provided with 2 n-channel transistors 64a and 64b, and a reference circuit including a pnp transistor 63 parasitized in a CMOS integrated circuit and a resistor r. An output current is taken out in the same manner as described above.
FIG. 4(c) illustrates a current source circuit using a thermal voltage for a reference, which circuit is different from the circuit of FIG. 4 (b) as to usage of two transistors 63a and 63b, which differ in current densities of the emitters, in the reference circuit.
Furthermore, FIG. 4(d) shows a current source circuit using a band gap voltage for a reference (P. R. Grey and R. G. Mayer, cited above, pp. 310). This circuit is formed by adding a fine adjusting circuit for adjusting temperature characteristics to the circuit shown in FIG. 4(c). This fine adjusting circuit includes a transistor 65, a resistor ra, an operational amplifier 66 which subtracts a voltage drop across a feed back resistor R receiving an output current Io from a voltage drop across the transistor 65 and the resistor ra, and an output transistor 67 controlled by an output of the operational amplifier 66. In this case, the output current Io is a so-called sink current, which is absorbed from a load.
As described above, the prior art current source circuits can output a reference current which is not affected by the variation of a power supply voltage and has considerably small temperature dependence, though some difference may exist among the circuits. But, since many constituent elements are used in each circuit, there is a problem that a large chip area is required for incorporating the constituent elements into an integrated circuit. That is, 5 to 6 MOS transistors, 0 to 3 bipolar transistors, and 1 to 3 resistors are required in the current source circuits in FIGS. 4(a) to 4(d). Therefore, the chip size becomes large and the cost becomes high in case of incorporating a plurality of the circuits at the required places in an integrated circuit.
As a simplest constant current element, a depletion type MOS transistor is conventionally utilized in a current saturation region. Since an n-channel MOS transistor can be used simply by connecting a gate with a source, the circuit configuration is much simplified. However, it has considerably large temperature dependence, by which a current value to be constant changes by a ratio of about 1.7:1 in a range of 0.degree. to 150.degree. C. Of course, this element can not be used in a circuit in which temperature dependent instability of the current causes problem.
Furthermore, in some cases, a constant current has to be generated, which has not only no temperature dependence but also a predetermined temperature coefficient, though not affected by the power supply voltage. For examples, when a reference voltage is generated by using a forward voltage of a diode, a negative temperature coefficient of the diode is canceled with a constant current having a positive temperature coefficient. Or, a temperature error of a detected signal of a sensor etc. is compensated by using a constant current having a positive or a negative temperature coefficient as the case may be.
In view of the foregoings, an object of the present invention is to provide a circuit, as simple as possible, which facilitates generation of a constant current having no temperature dependence or a predetermined temperature coefficient without influence of variation of the power supply voltage.